The invention relates to a lifting pin and more particularly to a system of lifting a heavy object such as a concrete body to be placed in an intended location. One specific use in which the invention is seen to good effect is in the placement of segments to build up a tunnel lining. For convenience the invention will be described with reference to that particular use that the invention is not to be limited thereby.

It is known to cast a threaded socket in a concrete segment and to locate a threaded pin in that socket. The pin comprises a shank which is received in the socket and a head which can be engaged by the end of an articulated arm mounted on a crane or the like by which the concrete segment can be lifted into place. The arm is disengaged following which the pin is removed and then grout can be pumped through the socket to fill a space behind the segment, e.g. the annulus between the tunnel lining and the tunnel bore wall. Unfortunately, this method of lifting is unreliable because the pin can unthread itself from the lifting mechanism especially when the concrete segment tends to rotate. Clearly this has safety implications.

It is one object of this invention to provide a system for lifting which is of improved security and has other advantages.

According to the invention in one aspect there is provided a method of lifting a concrete body for placement, e.g. as a segment of a tunnel lining, the method comprising

(i) engaging a lifting pin in a threaded socket therefor in the body, the pin having a head and a threaded shank, the socket and the pin having complementary threads shaped to cause the pin to interengage with the socket so as to enable the pin to engage the body for lifting and the threads of the pin being shaped to prevent unintentional threading of the pin from the socket;

(ii) engaging the head of the pin with a lifting device and then moving the body to the intended place, separating the lifting device from the pin and

(iii) releasing the pin from the socket.

Preferably the threads of the threaded portion comprise sloping side walls, one of which is relatively steeper and longer than the other. The threads are preferably arranged so that the lead in face of each thread, i.e. the one which leads into the socket, is the shorter and steeper and so presents minimal resistance whereas the trailing face presents are larger area for the subsequent unthreading. Preferably the faces are inclined so

that the pressure generated at the trailing face is about thre times as great as at the leading face. Preferably the thread are in a helix. Preferably the threads are shaped so that ther is a three point start for speed of insertion of the pin in th socket. The shank may include a frustoconical portion betwee the threaded portion and the head of the pin to engage with complementary portion of the body.

The threaded socket is preferably formed of Nylon or like plasti and has side flanges to anchor the socket in the concrete body. When the socket is cast in the concrete body, the concrete wil fill the space behind the threads so providing deep tapere shoulders of set concrete. As a result of the interengagement o the threads of the pin and those of the socket and the concret shoulders behind those socket threads, the pin can be used t lift a heavy concrete body. A pin of the invention can be use to lift a heavy concrete body and the socket can be used fo other purposes, e.g. grouting.

The head of the pin may be generally spherical and have sockets to receive a bar used to rotate the pin out of the socket.

Most preferably the method of lifting includes the step of locating the hood of a lifting means over the head of the pin, the hood containing a clamping device actuable to engage the head of the pin, and engaging the clamping device with the pin.

Preferably the clamping device comprises an upper bulb like body surrounded by a plurality of segmental fingers, the lower portion of the fingers being shaped to engage the head of the pin, and actuating cam means to urge the fingers to engage the head of the pin. Preferably the method includes the step of actuating the cam means from outside the hood.

In another aspect the invention provides a lifting pin comprising a head and a shank, the shank having a threaded portion, the threads being arranged in a helical path, the walls of the threads being shaped to prevent unintentional unthreading from a socket for the threaded shank and each having a relatively steep lead in angled face and a relatively shallow trailing angled face and the head of the pin being shaped for lifting.

In order that the invention may be well understood it will now be described with reference to the accompanying diagrammatic drawings, in which

Figure 1 is a side elevation of one embodiment of a lifting pin according to the invention overlying and partly in a socket therefor set in a concrete segment;

Figure 2A is an elevation partly in section of a lifting device about to engage the pin of Figure 1 and Figure 2B is a sectional view taken along lines II-II on Figure 2A;

Figure 3A corresponds to Figure 2A showing the device in clasping engagement with the pin and Figure 3B is a sectional view taken along lines III-III on Figure 3A; and

Figure 4A is a plan view and Figure 4B a vertical section of a plug used to seal the socket shown in Figure 1.

The lifting pin 10 comprises an elongate steel body comprising a spherical head 11 on top of a neck 12 mounted on an annulus 13 having side flats 14. The pin may be made in any convenient way, for example by being machined from a length of bar. Below the annulus is a frustoconical portion 15 beneath which is a helically threaded shank 16. The threading is of helical form, each thread 17 comprising one relatively longer side wall 17A and one relatively shorter sidewall 17B. The ratio of the side walls may be selected so that there is a steep lead-in angle (side wall 17B) and a relatively shallower trailing angle (side wall 17A) ; the ratio may be about 3:1. The thread is a three point start for speed of insertion into a socket 20.

The socket 20 is cast in the body of concrete 21. The socket 20 has a conical base portion 22 having an annular anchoring rim 23 at the lower face 21B of the concrete body. The top portion 24 of the socket is funnel shaped, having an annular rim 24A set in the upper face 21 of the body 21. The intermediate portion 25

of the socket 21 is threaded in a helix to complement that of the shank 16 of the pin 10. Because the socket 21 is cast in the concrete shoulders of set concrete are present behind the threads of the socket. The socket 20 is made of 2mm nominal thickness in a plastics such as NYLON.

In use, to lift the body 21 the pin 10 is threaded into the socket 20, the shape of the threads of the helix serving to engage the parts together and to lock them against unintentional unthreading. The body may then be lifted using the lifting device 30 shown in Figures 2 and 3. Because of the engagement of the threads of the shank and those of the socket and the concrete behind, the pin may be used to lift a heavy concrete body. The device comprises a hood 31 having a central socket 32 in the roof thereof for connection to an articulated limb or crane chain, neither of which is shown. A bulb-like body 33 depends from the centre of the hood and is surrounded by a plurality of segmental fingers 34. The inner walls of the fingers 34 have upper and lower arcuate portions 35 adapted to engage the bulb body 33 and the head 11 of the pin 10 respectively. The outer surface of the fingers has a depression 36 at a position about 2/3 of the height. Opposite cam members 37 are present along the side walls of the hood 30, and can be moved vertically by bars 38 projecting beyond the hood and travelling in slots therefor. The slots are inclined along a helical path. When the cam members 37 are lowered from the raised condition shown in Figure 2 to the

lowered condition shown in Figure 3 the fingers are brought together so that they engage the head 11 of the pin 10, the free ends 39 of the fingers resting against the wall of the neck 12. In this condition the concrete body 21 may be lifted, with confidence that the pin 11 is securely clamped to the lifting device 30 and is also held secure against any movement (axial or torsional) relative to the socket 30. After placement, the fingers 34 of the clamping device 30 are released from the head 11 of the pin 10 and the pin 10 is then unthreaded.

A plastics plug 40 shown in Figure 4 is then threaded into the socket 20, the plug corresponding in shape to the interior of the socket. The plug has a bore 41 and a blind end wall 42. The plug is used to block the socket 20 after pumping of grout behind the placed segment. The mouth of the plug may then be sealed.

The invention is not limited to the embodiment shown. The socket may comprise the threaded portion only and may not extend the full thickness of the body. One more sockets may be present per body but usually only one is needed. The plug may be serrated to destroy the socket. The body need not be a segment and may not be formed of concrete.